1. Field of the Invention
This invention relates to wire transport apparatus and to methods of wire transport, using opposed rollers which contact the wire at their nip to drive it along its direction of elongation.
2. Description of the Prior Art
EP-A-0138895 discloses a wire transport apparatus provided with rollers which form a gap at their nip between their peripheral surfaces for contacting the wire. One roller is driven and presses the wire against the other roller. A coupling pulley is coupled to the driven roller. A movable carrier is provided on which the driven roller and the coupling pulley are mounted. A flexible drive element runs around the running surface of the coupling pulley for driving the coupling pulley. A drive pulley drives the flexible element. This apparatus is further provided with a spring for exerting a specific additional normal force between the driven roller and the wire. Moreover, the drive pulley is suspended freely and coupled to a hydraulic servo-system in order to allow it to follow every movement of the driven roller.
In operation of this known apparatus, the spring force and the distance between the driven roller and the drive pulley is selected in such a way that the total normal force resulting from the spring force, from any pre-tension in the drive element and from the driving tensile force in the drive element is intended to provide slip-free conveying. However, when conveying a steel wire, it is found in practice that, in order to prevent slip, the normal force has to be increased after only a brief period of use. This is done by increasing the spring force and/or the pre-tension in the drive element. This contribution to the normal force is made so large that slip-free wire conveying is accomplished even under the highest opposing resisting force acting on the wire from outside the apparatus. Since this contribution to the normal force is continuously applied and is roughly constant, it does not diminish correspondingly with a lower resisting force acting on the wire. Consequently the normal force for each resisting force which is smaller than the maximum is unnecessarily high. This excess of normal force leads to accelerated wear so that slip soon occurs and the contribution to the normal force has to be increased once again. In this way, at a coefficient of friction of 0.3 and under a resisting force of 10% of the maximum resisting force, the normal force quickly attains more than 20 times the normal force needed, this being a problem that the known wire conveying apparatus was supposed to prevent. The coefficient of friction is defined here as the maximum friction force occurring with a normal force exerted perpendicularly on friction surfaces at which the driven roller and the wire touch each other, divided by the normal force.
FR-A-2294117 illustrates a different form of wire transport apparatus, in which a driven roller of a wire-contacting roller pair is co-axial with a drive roller which engages a conical driving member. The driven roller and drive roller are mounted on an arm, on an axis transverse to the arm, which can swing for adjustment of the position of the drive roller on the conical member, in order to vary the drive speed.